The tremendous growth of opto-electronic device manufacturing has raised the need for high performance micro and nanopositioners, which are used in assembly and alignment. The static and dynamic performance of micro and nanopositioners depends in part on the quality of operation of their controllers, which in turn depends to a significant degree on the accuracy of the kinematic and dynamic mathematical models upon which they are based.
The utilization of microelectromechanical systems (MEMS) for nanotechnology research and nanomanufacturing has a number of critical applications. The combination of MEMS and nanotechnology, however, presents a number of new challenges not experienced in more common MEMS applications in sensors and telecommunications. Most importantly, precision motion control of MEMS actuators is critical as resolution, accuracy, and repeatability are expected to be on the order of nanometers.
Nanopositioners equipped with nanoprobes are devices that can precisely manipulate nano-scale objects. The sizes of the nanopositioners used for this work are a few hundreds of micrometers. Motion range is 25 μm to 50 μm and desired step resolution is a few nm. The nm resolution opens the possibility to control nano-objects, such as nano wires and biological or chemical building blocks. These requirements constrain motion control law specifications in terms of the system precision and dynamic performance. A displacement sensor is a critical component in controlling the motion of a nanopositioner.
Displacement sensors, which are known in the art, can be embedded into the devices along the axes of the actuators, eliminating Abbe sine displacement measurement error and improving the accuracy capability of the nanopositioner. Designing a displacement sensor that can be embedded in an MEMS device is extremely difficult due to the small size of the devices, which typically have an external dimension of 1 mm to 3 mm and are made of a single crystal of silicon.
One example of a displacement sensor is disclosed by Osami Sasaki and Takamasa Suzuki in Interferometric Displacement Sensors Using Sinusoidal Phase-Modulation and Optical Fibers (Advanced Materials and Devices for Sensing and Imaging II, Proceeding of SPIE, Vol. 5633 (2005)) (hereinafter Sasaki et al.). The displacement sensor disclosed by Sasaki et al. generates coherent electromagnetic radiation using a laser diode coupled to an optical fiber through an optical isolator and lens. The injection current to the diode is modulated with a sinusoidal signal of controlled amplitude. The electromagnetic radiation is distributed to two optical fibers using a coupler. A portion of the electromagnetic radiation traveling through the first optical fiber is reflected by the end face of that fiber and forms the interferometer reference signal, which travels back through the optical fiber and falls on a photodetector. Another portion of the electromagnetic radiation traveling through the first optical fiber is reflected by the object (target) surface and propagates back through the optical fiber and falls on the photodetector. The photodetector detects the interference signal of the two waveforms.
The electromagnetic radiation traveling through the second optical fiber is directed to a photodiode that detects the intensity of the laser diode signal.
In addition, the Sasaki et al. technique requires that a feedback controller be used in order to maintain the amplitude of the laser diode current α=(λ02/4βP), where P is the distance between the end face of the first optical fiber and the object (target) reflecting surface, λ0 is the mean value of the modulated electromagnetic radiation wavelength, β is the modulation efficiency of the electromagnetic radiation source. If that condition is satisfied, the distance P is given by P=α(λ0/4π), where α is an angle determined by data obtained from the interferometric signal.
The amplitude of the laser diode current makes the Sasaki et al. technique impractical for use in MEMS devices. Due to the size of the MEMS devices, the interferometric sensor distance P ranges from a few micrometers to 150 μm. For a typical value of β=0.0171 nm/mA, the required current amplitude for a λ0=1550 nm is α=234.16 mA, which is a high power requirement that is impractical.
It is desirable to have a displacement sensor that requires a low voltage and power supply.
It is desirable to have a displacement sensor which is on a micro/nano scale size and is capable of being embedded in a microchip device to integrate the electromagnetic radiation source and the controller electronics in the device.
It is desirable to have a displacement sensor that can be interfaced with macro scale components.
It is desirable to have a displacement sensor that has range of up to a few millimeters, target proximity from a few micrometers to 0 micrometers, and accuracy, repeatability and resolution of a few nanometers.
It is desirable to have a displacement sensor that is non contact and has no moving components.
It is desirable to have a displacement sensor that is inexpensive to manufacture.